The present invention relates to a method for reducing odoriferous and/or toxic residual monomers, particularly residual ethyl acrylate and/or acrylonitrile in an aqueous polymer dispersion.
Ethyl acrylate (EA) is a common and renewable monomer used in the production of polymers including resins, plastics, rubber, and denture material. It can be prepared by several industrial methods, for example, acrylonitrile can be reacted with ethanol using sulfuric acid as a catalyst to produce ethyl acrylate. It may also be prepared from acetylene, carbon monoxide and ethanol. Ethyl acrylate will readily polymerize upon standing and polymerization is accelerated by heat, light, and peroxides. However, ethyl acrylate has safety issues due to its flammability, toxicity, and high reactivity.
One favorable factor of detecting the harmful ethyl acrylate in the polymers is that ethyl acrylate has a very special and bad odor, greater than 2 ppm residual EA in the latex will result in seriously unpleasant odor, general chasing method not easy to remove the trace residual EA monomer.
Acrylonitrile (AN), is another commonly used monomer in polymer industry, such as such as styrene-acrylonitrile (SAN), acrylonitrile butadiene styrene (ABS), acrylonitrile styrene acrylate (ASA) and other synthetic rubbers such as acrylonitrile butadiene (NBR). It is also widely used in coating, adhesive, non-wave, paper, concrete application and contributes lots of performance value like bindering strength, tensile strength, solvent resistance and so on. But it is highly flammable and toxic, and the burning material releases fumes of hydrogen cyanide and oxides of nitrogen. The International Agency for Research on Cancer (IARC) classified it as a Class 2B carcinogen (possibly carcinogenic). Acrylonitrile increases cancer in high dose tests in male and female rats and mice. So, in the industrial application, especially in decorative coating, the residual acrylonitrile monomer is a big issue for human health.
Currently, most approaches for reducing the amount of residual monomers of EA and/or acrylonitrile in aqueous dispersant, focusing on chasing the residual monomers are not efficient and result in other negative effects in performance and bring additional VOC. These methods are disclosed in, for example, U.S. Pat. Nos. 5,087,676, 6,365,709, 6,433,132 and 5,994,457.
Other attempts include using carbonyl compounds and/or their reaction products in the after-treatment of aqueous polymer dispersions as disclosed in the following patent applications/patents.
U.S. Pat. No. 7332565B2 discloses a method for reducing the amount of residual monomer in aqueous polymer dispersions by post-treatment with an initiator system comprises after-treating the aqueous polymer dispersion with addition of an initiator system essentially comprising an inorganic salt of persulfuric acid, methyl ketone, and catalytic amounts of a metal ion as optional which is able to exist in a plurality of valence states.
This method is efficient for reducing residual EA and/or AN monomers to a relatively low level for reducing the odor and toxicity, but for very low threshold EA and high toxic monomer AN, it is difficult to remove completely. Also, less residual monomer need more initiator, it will bring some negative results for performance compromising such as scrub resistance, flexibility and contribute lots of VOCs for the aqueous dispersant.
There is still a desire in the coating area, especially, in the architectural coating area, to find a new method of reducing the residual monomer of EA and/or acrylonitrile in the aqueous polymer dispersion. Therefore, the use of EA and/or acrylonitrile in the coating area could be developed and worries to the toxic and harmful problems of these residual monomers are no longer necessary. This new method is very simple, efficient and environmentally friendly, most important, the cost is low.